How do I waterproof a bathroom floor before tiling in Calgary?

Waterproofing a bathroom floor before tiling requires applying a continuous waterproof membrane over the entire substrate, ensuring water cannot reach the subfloor and cause structural damage. In Calgary's climate, where chinook-driven temperature swings cause expansion and contraction in building materials, proper floor waterproofing is especially important because movement can open hairline cracks in grout and allow moisture to penetrate.

The process begins with preparing the substrate. Your subfloor should be clean, level, and structurally sound — typically 3/4-inch plywood in Calgary homes. Any squeaks, soft spots, or unevenness need to be addressed first. For bathroom floors, install cement backer board (such as Durock or HardieBacker) over the plywood using thinset mortar and backer board screws spaced every 8 inches. This costs roughly $1.50–$3 per square foot for materials in Calgary. Never tile directly over plywood, and never use regular drywall or green board as a floor substrate in wet areas.

Once the backer board is down, apply your waterproofing membrane. The two most common options for Calgary bathroom floors are liquid-applied membranes like RedGard and sheet membranes like Schluter Kerdi. For a liquid membrane, apply two coats with a roller or brush, allowing the first coat to dry completely before applying the second. Each coat should be applied in alternating directions for full coverage. The finished membrane should be a uniform colour with no thin spots — you should not be able to see the substrate through the membrane. RedGard runs about $1–$3 per square foot for materials. For a sheet membrane like Schluter Kerdi, apply it over unmodified thinset, pressing firmly to ensure full contact with no air pockets. Kerdi runs $3–$6 per square foot for materials.

Pay special attention to transitions and penetrations. Where the floor meets the walls, carry the membrane up at least 4–6 inches to create a "bathtub" effect. Around toilet flanges and any floor drains, use compatible pre-formed membrane pieces or additional coats of liquid membrane to ensure a watertight seal. These transition areas are where most waterproofing failures occur. In Calgary, where chinook temperature cycling stresses joints and seams, using manufacturer-recommended transition accessories rather than improvising is critical.

Under the Alberta Building Code, bathroom floor waterproofing in shower areas is required, and a Safety Codes Officer will inspect this work if you have pulled a permit for plumbing or structural modifications. Even in areas outside the shower where permits may not be required — such as the general bathroom floor — adding a waterproof membrane is a best practice that protects against splashes, overflows, and slow leaks from toilet seals or supply lines. The cost of waterproofing an average Calgary bathroom floor runs $500–$1,500 including materials and labour.

This is a project best left to a professional. While applying a liquid membrane may seem straightforward, ensuring complete coverage with no pinholes, properly detailing transitions, and achieving the correct membrane thickness requires experience. A failed waterproofing job may not reveal itself for months or even years, by which time water damage to the subfloor and joists can cost $5,000–$15,000 or more to repair. A qualified bathroom renovation contractor will ensure the membrane is applied correctly the first time. Calgary Bath Remodeling can match you with local contractors experienced in proper waterproofing techniques through the Calgary Construction Network.

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Looking for experienced contractors? The Calgary Construction Network connects homeowners with qualified professionals:

• Onsite Contracting and Electrical Services
• The Original Workshop
• Canadian Closet
• Greenstone landscaping solutions
• Calgary Custom Concepts